CaraLombardo

Activist investor Elliott Management Corp. has taken a big stake in Nielsen Holdings PLC and plans to push the TV-ratings company to sell itself.

The New York hedge fund owns more than 8% of Nielsen
NLSN, +3.44%
, worth at least $640 million, people familiar with the matter said Sunday. Elliott, which has been launching campaigns at a blistering pace this year, could unveil the new effort as soon as Monday. Multiple private-equity firms already have expressed interest in Nielsen, the people said.

Nielsen, which measures how people shop and consume media such as radio and TV, has been hurt by a rapidly changing retail environment. The company has a market value of almost $8 billion and is the S&P 500’s third-worst performer in 2018, with its shares off around 40% so far this year.

Nielsen’s stock price tumbled 25% in a single day last month after the company reported a weak second quarter and lowered its revenue estimates for the year. Chief Financial Officer Jamere Jackson said in July the second quarter “was one of the most challenging quarters for our business in over a decade.” He cited challenging conditions for consumer-packaged-goods clients, which hurt its “buy” business, which measures retail and consumer behavior.

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